You must "View Config File" all of it dependencies and check to see if they all are trainzbuild 2.6 or below.
If they are above trainzbuild 2.6 ... they must be backdated in "Edit In Explorer" to the trainzbuild line of 2.6
Any asset backdated will not be recognized as backdated, until you delete the assts.bku file in cache, and then delete the assets.tdx file in TRS2006 folder.
Shutting down Trainz ... restart the CMP, and press "Manage Content", and then walk away from your PC ... let it rebuild undisturbed for @ 30 min.